The present exemplary embodiment relates generally to electrophotographic printing. It finds particular application in conjunction with controlling the tonal reproduction curve (TRC) and maintaining toner concentration (TC) within the latitude range of the electrophotographic print system, and will be described with particular reference thereto. However, it is to be appreciated that the present exemplary embodiment is also amenable to other like applications.
In copying or printing systems, such as a xerographic copier, laser printer, or printer, a common technique for monitoring the quality of prints is to artificially create a “test patch” of a predetermined desired density. The actual density of the toner or ink in the test patch can then be optically measured to determine the effectiveness of the printing process in placing this toner on the print sheet. In the case of xerographic devices, the surface that is typically of most interest in determining the density of toner thereon is the charge-retentive surface or photoreceptor, on which the electrostatic latent image is formed and subsequently, developed by causing toner particles to adhere to areas thereof that are charged in a particular way. In such a case, the optical device for determining the density of toner on the test patch, which is often referred to as a “densitometer,” is disposed along the path of the photoreceptor, directly downstream of the development of the development unit. There is typically a routine within the operating system of the printer to periodically create test patches of a desired density at predetermined locations on the photoreceptor by deliberately causing the exposure system thereof to charge or discharge as necessary the surface at the location to a predetermined extent.
The test patch is then moved past the developer unit and the toner particles within the developer unit are caused to adhere to the test patch electrostatically. The denser the toner on the test patch, the darker the test patch will appear in optical testing. The developed test patch is moved past a densitometer disposed along the path of the photoreceptor, and the light absorption of the test patch is tested—the more light that is absorbed by the test patch, the denser the toner on the test patch. Xerographic test patches are traditionally printed in the interdocument zones on the photoreceptor. They are used to measure the deposition of toner on paper to measure and control the tonal reproduction curve (TRC). Generally each patch is about an inch square that is printed as a uniform solid half tone or background area. This practice enables the sensor to read values on the tonal reproduction curve for each test patch.
The process controls that are generally monitored include developability, which is the rate at which development (toner mass/area) takes place. Developability is typically monitored (and thereby controlled) using infrared densitometers (IRDs) and by measuring toner concentration (TC) in the developer housing. As described above, IRDs measure total developed mass (i.e., on the imaging member), which is a function of developability and electrostatics. Thus, the developability cannot be determined using IRDs alone because the electrostatics of the imaging member also affects the mass of toner deposited on the imaging member by a developer device. Toner concentration is measured by directly measuring the percentage of toner in the developer housing (which, as is well known, contains toner and carrier particles). However, the relationship between TC and developability is affected by other variables such as ambient temperature, humidity and the age of the toner. For example, a 3% TC results in different developabilities depending on the variables listed above. Thus, maintaining toner concentration at a predetermined value does not ensure a desired developability.
Thus, in xerographic print engines, a TRC controller is critical to the image quality of the output. In recent years, in order to better control image quality of the output of a print engine, most middle to high end products have started to control TRC using a three-point target in TRC controller, i.e. solid, mid-tone and highlight. This is a change from the previous one-point target (solid only) or two-point target (solid and highlight). Adding the new point in the mid-tone area has added complexity to the TRC controller. Convergence of the electrostatic actuators by the TRC controller depends on factors such as the hardware, TRC control algorithms, and the environment and toner concentration in the development housing. Toner concentration is a critical factor in making the TRC controller work well and keeping the xerographic system within latitude. In the past some machines used a constant toner concentration target. Other machines used toner concentration target per environment change. For these machines, the toner concentration target used in toner concentration control is independent of TRC control. This strategy works most of the time, but there are situations where the TRC is “bent.”
A “bent” TRC is illustrated in FIG. 1. TRC control generally provides uniform gray scale development and effective translation of halftones, highlights, and shadow details, as well as mid-tone densities. The control stability of all the density levels on the TRC makes photographic reproductions and other halftone documents invariant from machine-to-machine and copy-to-copy. Referring to FIG. 1, the TRC is shown in terms of a measure of whiteness (L*) versus the toner area coverage (Cin.) of developed image fill patterns. L* represents the differential response of the human eye to a developed image and is used as a metric for density variation. Since L* is non-linear in terms of density, density information for values of Cin. are converted to L* as explained in U.S. Pat. No. 5,436,705 at column 5, lines 56–68, and column 6, lines 1–11. The variations in the L* values shown in FIG. 1 are typically controlled to a standard deviation of plus or minus 2 units or 2 sigma-limits. The standard deviation is indicated graphically by a space defined between the two opposing solid lines adjacent to the bent TRC. The upper and lower boundaries are used to decide if image quality is satisfactory. If the image quality is above the upper boundary or below the lower boundary, it will not pass the set-up mode.
In the situation where the TRC is “bent,” (i.e., the solid patch is too light and the mid-tone patch is too dark or the solid patch is too dark and the mid-tone patch is too light), without moving toner concentration per the TRC controller, there are typically two choices: compromise the TRC of the output prints or drive the electrostatic actuators to the point of divergence. When the TRC is “bending” and the toner concentration does not move, this will normally drive electrostatic actuators to the point of divergence in order to keep the TRC within tolerance, or the TRC will be compromised if the actuators are restrained from moving. When the actuators diverge, the TRC of the output will either be out of specification or the TRC controller may fault and cause the machine to cycle down. The end result will be that customers will either have to make a service call or they will have compromised image quality.
Thus, there is a need for an improved method and system for using a TRC controller to change the target of the toner concentration sensor and use toner concentration as an active actuator to compensate for the bending of the output TRC is needed. Such a method and system would change toner concentration properly, so that the TRC bending issue will be resolved without causing any system latitude issues. The improved method and system would change toner concentration properly in both amplitude and direction, based on either the level of the electrostatic actuators or the difference between the target and readings of the relative reflectance (RR) from the black toner area coverage (BTAC) sensor. This improved method and system would also ensure that the TRC will be controlled without driving the electrostatic actuators to divergence and the toner concentration will be maintained within the range of the xerographic system latitude requirements.